The present invention relates to a method for purifying carbon fluorides, particularly to a method for removing a non-bonding fluorine which are remaining in carbon fluorides or regulating an amount thereof.
Hitherto, carbon fluorides are widely used as active materials for batteries by utilizing their electrochemical properties, or as solid lubricants by utilizing their excellent lubricity, or also as water and oil repellents. The carbon fluorides are prepared by reacting carbon materials such as natural or artificial graphites with fluorine gas. It is known that the carbon fluorides can have two chemical structures, i.e. (CF).sub.n and (C.sub.2 F).sub.n, depending with starting materials or preparation conditions. Also, it is known that unreacted fluorine which does not bond to a carbon atom is present in the resulting carbon fluorides and, that the unreacted fluorine gives various affects.
For example, when carbon fluorides containing a large amount of this non-bonding fluoride are used as active materials for batteries, the non-bonding fluorine causes a Li surface of collector to corrode and increases an internal resistance, which results in lowering of battery characteristics. In addition, an initial discharge voltage sometimes rises to which is a high voltage, e.g. 7.0 V, to give damage to IC circuits and the like.
Accordingly, it is said that the non-bonding fluorine in carbon fluorides should be removed therefrom. As used for active materials for batteries, however, too small amount of the non-bonding fluorine is not desirable, because a discharge voltage, particularly an initial discharge voltage becomes lowered. Therefore, a purification method by which an amount of the non-bonding fluorine in carbon fluorides can be controlled is desired, but prior purification methods are not necessarily satisfactory.
For example, Japanese Tokkyo Kokoku No. 36790/1981 discloses a treating method with mixed solutions of alcohols and strong acids or strong alkalis. The treating method has problems that strong reagents such as strong acids or strong alkalis are used, that post steps such as water linsing, filtration and drying are required after the treatment, and that an amount of the remaining non-bonding fluorine is difficult to control.
Japanese Tokkyo Kokoku No. 13930/1988 discloses a treating method with ammonia gas. The method, however, has a high danger due to the use of combustible ammonia gas. Moreover, NH.sub.4 F produced by the reaction happens to be contained in the treated carbon fluorides as a contaminant. In order to remove NH.sub.4 F, a post treatment in which NH.sub.4 F is sublimated is required. As mentioned above, this method has problems to be improved.
The present invention relates to a safe and easy purification method by which a delicate control can be achieved.